(a) Fields of the Invention
The present invention relates to methods for fabricating a semiconductor device. In particular, the present invention relates to methods for fabricating a semiconductor device which includes a trapping film for charge accumulation and a bit line both provided for a MONOS nonvolatile semiconductor memory device.
(b) Description of Related Art
MONOS (metal-oxide-nitride-oxide-semiconductor) nonvolatile semiconductor memory devices are nonvolatile semiconductor memory devices in which charges are accumulated in an ONO film of a stacked structure made by sequentially stacking silicon oxide, silicon nitride, and silicon oxide. Until now, various types of MONOS nonvolatile semiconductor memory devices have been proposed. Among them, an attention-getting one is a nonvolatile semiconductor memory device which has: a bit line formed in a semiconductor substrate; an ONO film formed on a channel region; and a word line formed above the bit line to perpendicularly intersect the bit line, and which stores information by accumulating charges locally in the ONO film (see, for example, Japanese Unexamined Patent Publication No. 2001-77220). The reason why this device receives attention is that it is suitable for high integration, performance enhancements and voltage reduction.
Hereinafter, with reference to FIGS. 8A to 8G, description will be made of an example of conventional methods for fabricating a nonvolatile semiconductor memory device in which bit lines are formed in a semiconductor substrate.
Referring to FIG. 8A, first, a trapping film 102 of an ONO film is formed on a semiconductor substrate 101 of a first conductivity type.
Next, as shown in FIG. 8B, on the trapping film 102, a photoresist pattern 103 is formed which defines a layout of a plurality of bit lines.
As shown in FIG. 8C, upper portions of the trapping film 102 are removed using the photoresist pattern 103 as a mask. Note that in this removal, lower portions of the trapping film 102 are left, but those portions of the trapping film 102 may be fully removed to expose the semiconductor substrate 101.
Subsequently, as shown in FIG. 8D, using the photoresist pattern 103 as a mask, impurity ions of a second conductivity type are implanted into the semiconductor substrate 101 through the lower portions of the ONO film 102. Thereby, a plurality of diffused layers 104 of the second conductive type are formed in regions of the semiconductor substrate 101 located below openings of the photoresist pattern 103.
Next, as shown in FIG. 8E, the photoresist pattern 103 is removed and then, as shown in FIG. 8F, the diffused layers 104 are subjected to enhanced oxidation. Thereby, an upper portion of each diffused layer 104 and both ends of the trapping film 102 located toward each opening are formed with insulating oxide films 105 and 106 of silicon oxide, respectively. Simultaneously with this, the implanted impurity ions are activated to form bit lines made of the diffused layers 104.
As shown in FIG. 8G, a polysilicon 107 having conductivity is deposited on the ONO film 102 and the insulating oxide films 105 to form an electrode (a word line) made of the conductive polysilicon 107. In the manner described above, a nonvolatile semiconductor memory device is completed.